More testing reveals that the font rendering problem (blank blocks) is only presented in RN preview mode and not when the exported html is opened with Chromium or Firefox. I think you should take a look at the rendering engine.

Also, as far as the font substitution concerns, DejaVu Sans is an adequate substitution for Ubuntu whenever there is a missing character as the two are almost indistinguishable at a glance. Therefore, this font line should do nicely: "font-family: Ubuntu, DejaVu Sans, Arial, sans-serif;".

summary:

- RN should not be using Ubuntu typeface by default+ Add font selection option (was "RN should not be using Ubuntu typeface+ by default")

You guess it right. The font rendering in midori is identical to RN. The Ubuntu/Dejavu combination irons out visual inconsistency for missing characters but the Japanese font is still not substituted properly.

Here is the same page as in the previous comment rendered in RN and midori with that font line.